Using one shop's gift card in another

I have both Tesco and Asda gift cards in my wallet. Yesterday I produced an Asda gift card at a Tesco by accident. The shop assistant hesitated a bit but swiped it anyway. I was then asked to sign a slip, even though there is no signature on the Asda gift card, and the payment went through. The receipt shows something like "Intl Maestro".

This got me thinking: these days many shops contracted out their gift card management to third-parties which, presumably, use some kind of standard format. Sometimes, they are simply prepaid debit cards in disguise. This might allow one shop's gift card to work in other shops, perhaps.

Anyone has any experience?